To better understand the invention, U.S. Pat. Nos. 5,481,116, 4,980,562 and 4,922,106 are incorporated herein by reference as background.
The steady increase of wafer size within the semiconductor industry has resulted in ion implanters using relatively high scan angles to cover the wafer. Undesirably, high scan angles tend to cause dose non-uniformities. Dose non-uniformities also increase due to processing efficiencies and plasma and system peculiarities that occur during operation.
At the same time, industry demands for ion dose uniformity make it desirable to reduce or eliminate dose non-uniformities. The prior art has attempted to address these issues. In U.S. Pat. No. 5,481,116, for example, an elaborate scanning magnet system is described to reduce a 0.5% drop in beam current at the center of the scan. The ""116 patent also describes the use of fourth order polynomials in the magnet pole shapes, presumably to obtain a scan with uniform dose and uniform implant angle. The ""116 patent attempts to achieve precise uniformity without feedback control, which is difficult, expensive, and ultimately insufficient if system operation is not correctly predicted.
In U.S. Pat. No. 4,922,106, a system is described for correcting non-uniformities in dose by measuring the beam at the wafer plane and adjusting the scan pattern to obtain a uniform dose. The teachings of the ""106 patent thus require a degree of control over the scan that are difficult to achieve when magnets are used for scanning. The ""106 patent also suffers from a limited dynamic range: reducing the dose at a point requires scanning the beam faster and faster without practical limits as to how fast a capacitive or inductive load may be driven. These problems are exacerbated when the ion implantation system uses a small beam current to fill a wafer area that was slightly under-dosed in a prior mechanical scan.
It is, accordingly, one object of the invention to provide an ion beam scanning system and method that reduce or eliminate the afore-mentioned problems. Other objects of the invention include utilizing beam current control during one axis of the electrical scan. Still other objects of the invention are apparent within the description which follows.
In one aspect, the invention provides uniform ion dose at the wafer position by varying the current of the ion beam synchronously with the scan. In the preferred aspect, the beam is scanned by a linear scan, but beam scan position information is sent from the beam scan electronics to the beam control circuit connected with the ion source; this information transfer preferably occurs over a fiber optic link to cross the high voltage between the two sets of electronics. Preferably, at initiation, the beam current is held constant and a Faraday cup is scanned across the beam to measure the variation of dose with scan position. A beam versus scan position waveform is calculated to correct the variation in dose; and the waveform is then loaded into a memory in the ion beam control circuit. The ion beam control circuit then varies the output of the ion source synchronously with the scan to adjust the dose as a function of scan position, as determined by the waveform. If necessary, repeated measurements and waveform calculations can be made until the dose is uniform.
Accordingly, in the preferred operation of the invention, the ion beam is controlled by a mirror electrode that adjusts the arc current at high speeds on the order of 0.05-0.1 MHz. Such speeds are 10-50 times larger than the scan speeds of about 1 ms per scan. Accordingly, the invention provides fine adjustment of beam current during the scan to achieve dose uniformity throughout the scan. Alternatively, in another aspect, the arc current is adjusted by another technique such as modulating the arc voltage.
In addition to obtaining a uniform dose along the electronic scan axis, in another aspect, feedback from a Faraday cup is used during implantation to vary the ion source current. This feedback is used to correct variations in beam current at the wafer produced by photoresist outgassing or other effects that in turn cause the beam transport or wafer or ion source efficiency to vary during an implant.
In one aspect, the invention provides a method of uniformly implanting a wafer with an ion beam, including the steps of: generating an ion beam from an ion source; determining a first ion dose versus scan position for the ion beam scanned across a target location; and adjusting source current according to the first ion dose versus scan position to adjust the ion dose when the ion beam is scanned across the target location, and as a function of scan position, such that a substantially uniform ion dose is generated at the target location.
In another aspect, the method can include the further step of positioning a wafer at the target location, and ion implanting the wafer with a substantially uniform dose of ions while the ion beam is scanned across the target location.
In a preferred aspect, the step of determining ion dose versus scan position utilizes a Faraday cup scanned approximately across the target location.
In still another aspect, the first ion dose versus scan position is formed and loaded as a waveform into memory; the waveform is then used in adjusting source current so as to provide the substantially uniform dose.
Preferably, an arc current controller is used to adjust the ion source current.
In another aspect, the method includes the further steps of measuring end of scan ion dose, and varying a magnitude of the dose versus scan position to further reduce dose non-uniformities.
In yet another aspect, the invention provides a system for producing a substantially uniform ion dose on a wafer. An ion source, driven by applied current, is used to generate an ion beam. An ion scanner is used to deflect the beam along a scan direction and across the wafer. An ion detector is used to sense current density of the ion beam scanned across the wafer. A controller is used in feedback with the ion scanner to adjust the applied current as a function of information characterizing the position of the ion scanner to achieve substantially uniform ion dose across the wafer.
Preferably, the system includes solid-state memory coupled to the controller to store an ion dose versus scan position waveform. In one aspect, at each scan, or periodically, the controller and detector cooperate to scale the waveform to ensure the substantially uniform dose; in this aspect the detector is used to measure ion does at the xe2x80x9cend of scanxe2x80x9d at a location adjacent to the target wafer. In one specific aspect, the detector is a Faraday cup.
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.